Zi-yan Yu

Larger magnetocaloric effect in two-layered La1.6Ca1.4Mn2O7 polycrystal

The magnetocaloric effect in La1.6Ca1.4Mn2O7 polycrystal with the layered Sr3Ti2O7-type perovskite structure has been studied. It is found that the magnetic entropy change of La1.6Ca1.4Mn2O7 is comparable to that of Gd, but smaller and more broadened than that of La0.8Ca0.2MnO3.

A bidirectional tunable optical diode based on periodically poled LiNbO 3

We propose a bidirectional tunable optical diode based on a periodically poled lithium niobate (PPLN) with defect. An acoustic wave propagates together with the light beam so that a collinear photon-phonon interaction happens, which affects the nonlinear optical processes in PPLN. The fundamental wave exhibits an optical diode effect, i.e., the light only may travel toward a single direction while the opposite way is isolated. However, the acoustic wave could be used to adjust the contrast of optical isolation from -1 to 1. A direction-optional operation is thus realized. Moreover, the advantages of our tunable PPLN optical diode are also discussed.

Polarization independent quasi-phase-matched sum frequency generation for single photon detection

Polarization independent sum frequency generation (SFG) is proposed in an electro-optic (EO) tunable periodically poled Lithium Niobate (PPLN). The PPLN consists of four sections. External electric field could be selectively applied to them to induce polarization rotation between the ordinary and extraordinary waves. If the domain structure is well designed, the signal wave with an arbitrary polarization state could realize efficient frequency up-conversion as long as a z-polarized pump wave is selected. The applications in single photon detection and optical communications are discussed.

Tunable broadband isolator based on electro-optically induced linear gratings in a nonlinear photonic crystal

We theoretically propose a broadband optical isolator based on second-harmonic generation in a one-dimensional quadratic nonlinear photonic crystal (NPC) with an embedded defect. An external electric field along the z axis is applied to modulate the NPC refractive index periodically. Complete optical isolation always could be reached with the help of an external field. Influences of the defect position and thickness are discussed. The spectral and power tolerances of the isolator also have been investigated and show high contrast within a wide wavelength range at different power levels.

Broadband and highly efficient quadratic interactions in double-slot lithium niobate waveguides through phase matching

We propose and study in detail a phase-matched quadratic optical interaction in a realizable 2D double-slot lithium niobate (LN) waveguide. As opposed to the single-slot waveguide, the field can be well confined in the nanometric nonlinear material, and it is also more flexible for birefringence and dispersion design. The proposed compact double-slot structure could not only achieve form birefringence phase matching but also effectively enhance the modal overlap integral and expand the working wavelength. The calculated results on second harmonic generation show an extremely large bandwidth of ~40 nm. The modal overlap integral up to 0.035 W(3/2)/V can be realized by optimizing the waveguide geometry, and it is much better than previous results on single-slot waveguides. Its temperature dependence is low--around 25 °C. The geometry is practical considering the current micromachining technique of LN.

Electro-optic tunable optical isolator in periodically poled LiNbO3

We proposed a design of a tunable optical isolator. The electro-optic (EO) effects in periodically poled lithium niobate (PPLN) with a defect were studied. EO effects and second harmonic generation (SHG) allow three-wave coupling in this nonlinear optics process. A defect is inserted in an asymmetrical position in PPLN to break the reciprocity of the fundamental-wave transmission in two reversed propagating directions. We numerically demonstrated that the contrast of this optical isolator can be tuned from −1 to 1 by simply changing the external dc electric field. With EO effects, the position of the defect can be designed more flexibly maintaining a high contrast of the optical isolation.

How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells

Radial junction (RJ) architecture has proven beneficial in boosting light harvesting and fast carrier separation in thin film solar cells. While a comprehensive understanding of the detailed absorption distribution and light incoupling mechanism within such a 3D RJ configuration remains largely unexplored. Taking hydrogenated amorphous Si (a-Si:H) RJ solar cells as an example, we here address in both experimental and theoretical manners the impacts of tilting and spacing configuration on the light absorption and external quantum efficiency (EQE) responses. A nice agreement between the calculated and experimental EQE responses indicates that the light harvesting realized within RJ thin film solar cells is quite robust against geometric variations and shadowing effects. Following the concepts of optical fiber injection, we have been able to single out the contribution arising solely from a resonant-mode-incoupling into the RJ cavities against a sidewall scattering incidence scenario. These results provide insightful viewpoints as well as practical guides in developing a new generation of high performance RJ thin film solar cells.

POLARIZATION INSENSITIVE QUASI-PHASE-MATCHED SECOND HARMONIC GENERATION

Polarization insensitive second harmonic generation (SHG) is proposed in an electro-optic (EO) tunable periodically poled Lithium Niobate (PPLN). The PPLN consists of four sections. External electric fields could be selectively applied to them to induce polarization rotation between the ordinary and extraordinary waves. If the domain structure is well-designed, the fundamental wave with an arbitrary polarization state can be frequency doubled efficiently in this special PPLN.

Aberration analysis and efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirec- tional optical subassembly BOSA modules is proposed and experimen- tally demonstrated. We analyzed the wavefront aberration coefficients of a typical BOSA. It was found that the 45-deg wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode to fiber coupling. We measured the BOSA efficiencies based on a series of different filters. For a typical 0.5-mm filter, 25% coupling efficiency im- provement was achieved by optimizing the filter parameters.

The efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirectional optical sub-assembly (BOSA) modules is proposed and experimentally demonstrated. We analyzed the wave-front aberration coefficients of a typical BOSA. It was found that the 45° wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode (LD) to fiber coupling. We measured the BOSA efficiencies based on a serial of different filters. For a typical 0.5 mm filter, 25% coupling efficiency improvement was achieved by optimizing the filter parameters.